A conventional synchronous machine has a rotor, which is excited with direct current, and a stator having a three-phase winding. Synchronous machines are used as a motor with a constant speed or else as phase shifters. They are also an important electrical machine for producing electrical energy. In this case, a distinction is made between permanently excited and separately excited synchronous machines which must be continuously supplied with an excitation current. This may be a direct current or a three-phase current depending on the type of excitation machine.
In order to control the phase currents in such a manner that a desired torque is produced with an efficiency which is as optimal as possible, exact information relating to the position of the rotor with respect to a stator winding is always required. In conventional synchronous machines, the rotational angle of the rotor is detected in this case using a sensor, for example using a resolver or a sine/cosine transducer.
However, such techniques increase the complexity of the overall system and reduce its reliability. The synchronous machine must therefore have a sensor which is installed in the rotor or is mechanically coupled to the latter, which results in increased costs and requires additional installation space for the sensor. The replacement of such a sensor is also associated with a considerable amount of effort. Furthermore, non-ideal sensor signals are intended to be compensated for and conceivable sensor errors are intended to be diagnosed as reliably as possible, on the other hand.
There is therefore the desire for sensorless detection of the position of the rotor of a synchronous machine, that is to say to replace the sensor with mathematical models, or by using physical effects.
The document EP 0 539 401 B1 discloses a method for controlling sensors of a synchronous machine, in which test signals are fed in and evaluated and which is based on the determination of an inductance which changes over the circumference of the rotor.